BARCELONA, Aug 31 (Reuters) - An experimental cholesterol-lowering drug from Sanofi and Regeneron Pharmaceuticals roughly halved the number of heart attacks and strokes in a clinical trial, researchers reported on Sunday.

The result is not conclusive, because the analysis was done retrospectively, but it provides the first evidence that targeting a protein known as PCSK9 could slash cardiovascular risks for millions of patients.

The injectable drug, alirocumab, is from a new class of medicines, which are also being developed by Amgen and Pfizer. They lower "bad" LDL cholesterol in a new way and are widely expected to reap multibillion-dollar sales.

The finding is likely to spur enthusiasm about the drugs, which could reach the market next year, although experts said it still needed to be confirmed in a much larger trial.

Sanofi and Regeneron said in July that nine big studies showed consistent LDL reductions with alirocumab, but details from four of these trials have only now been unveiled at the European Society of Cardiology annual meeting in Barcelona.

The encouraging cardiovascular data come from an interim safety analysis of one of these studies showing patients on alirocumab were less prone to a combination of cardiovascular events, including cardiac death, heart attack, stroke and chest pain requiring hospitalisation.

Both groups of patients got traditional anti-cholesterol statin pills in addition to alirocumab or a placebo. Among the alirocumab group, 1.4 percent of patients suffered a major cardiovascular event against 3.0 percent of those on placebo.

The ongoing 2,341-patient study, called Odyssey Long Term, is expected to conclude early next year but researchers said the early sign of efficacy was clearly positive.

"To have this result emerge so quickly in this study is very encouraging," said Jennifer Robinson, a cardiologist at the University of Iowa, who led the study.

No other drugmaker has previously released data suggesting reduced cardiovascular risk from PCSK9 inhibitors.

Patrick O'Gara of the Brigham and Women's Hospital in Boston and president of the American College of Cardiology said the finding was "biologically plausible" but the post-hoc or retrospective nature of the analysis necessitated caution.

"It's so much wished-for that we must be careful," he said.

WATCHING SIDE EFFECTS

Sanofi and Regeneron are in a fierce race with Amgen, which last week became the first company to file with regulators to sell its product evolocumab.

The French and U.S. partners hope to close the gap after paying $67.5 million last month for a voucher from BioMarin Pharmaceutical designed to assure alirocumab an expedited U.S. regulatory review.

It is unclear if the rival PCSK9 drugs will prove to be very different from each other but Sanofi's research head Elias Zerhouni thinks alirocumab should enjoy an early advantage from the preliminary indication of improved cardiovascular results.

That may help convince cost-conscious healthcare providers to pay up for an expensive new medicine before results from the large 18,000-patient trial are available in 2017 or 2018.

"It has to be confirmed by the big outcomes study but it is the first scientific indication that there is something other than a statin that potentially shows you can get a reduction in risk," Zerhouni told Reuters.

All four trials presented in Barcelona showed alirocumab cut LDL by around 50-60 percent after 24 weeks. Side effects included stuffy nose and upper respiratory tract infections.

Doctors and regulatory authorities are watching closely to see if cutting LDL so sharply may have an adverse effect on other parts of the body, notably the brain.

So far, there is no sign of this. There was a small non-statistically significant trend towards more neurocognitive problems in Odyssey Long Term but the other three studies showed alirocumab patients had fewer such issues.

PCSK9 drugs will be targeted first at people with a rare familial condition that raises their cholesterol dangerously, as well as statin-intolerant patients and those at high-risk of a heart attack who struggle to control LDL on current medicines.

Sanofi and Regeneron estimate this high-risk group totals around 21 million patients in the United States and Europe.

Whether the drugs, which will be offered in pre-filled syringes and auto-injector pens, will be used more generally depends on how their cost-effective they are deemed to be.

The companies say it is premature to talk about price but it is clear the antibody-based therapies will not be cheap, with Barclays analysts suggesting $6,000 a year as a possible price in the United States, with a lower price likely in Europe.